This invention relates to thermal insulation and, more particularly, to reusable, low density, high temperature cryogenic foam insulation systems and processes for their manufacture.
A pacing technology for liquid hydrogen fueled, high-speed aircraft is the development of a fully-reusable, flight-weight, cryogenic insulation system for propellant tank structures. Previously developed cryogenic tank insulation systems have employed low density polymeric foam materials with good success, however, they are not reusable. The current state of the art in insulation systems for flight-weight cryogenic containment vessels is a spray-on foam insulation (SOFI) system. This material is a rigid closed cell polyurethane foam which has excellent insulation qualities at low temperatures. This material is quite fragile and is often reinforced and/or coated to improve its durability with subsequent increases in density and conductivity. Also the material begins to decompose at 255.degree. F. and therefore is limited to a 175.degree. F. maximum use temperature. Materials of this type cannot withstand the temperatures generated by aerodynamic heating encountered during high speed flight and therefore require a separate thermal protection system to maintain the cryogenic insulation below its maximum use temperature.
The transition from current-art expendable launch vehicles to advanced liquid hydrogen fueled aircraft will require more efficient cryogenic insulation systems that are stronger and more thermally resistant to the hypersonic flight environment. Additionally, the cryogenic insulation systems must provide adequate thermal insulation to minimize fuel boil-off, prevent the liquefaction of air on the outer tank wall, and most importantly must be fully reusable.
Accordingly, it is an object of this invention to provide a reusable, light-weight cryogenic foam insulation system that can withstand cryogenic environments of -425.degree. F. while remaining thermally resistant to hypersonic flight environments of 400.degree. F.
A further object of this invention is to provide a cryogenic foam insulation system that prevents permeation of gases and liquefaction of air on the outer tank wall and minimizes fuel boil-off.
A further object of this invention is to provide a cryogenic foam insulation system for the inner wall of a cryogenic fuel tank structure that is impermeable to both air and the cryogenic fuel.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings which follow.